Canada is leading a campaign to oust the head of the UN Food and Agriculture Organization. In the corridors at an FAO Council meeting in Rome last week, Canadian officials were urging delegates to vote for Moise Mensah of Benin to replace the current director-general, Edouard Saouma, a Lebanese whose second term expires at the end of this year. They charge Dr. Saouma with mismanagement.

The FAO is second only to the World Bank as the largest provider of foreign aid within the UN family. The FAO has a two-year budget of $420 million and about $1 billion in pledges from donor nations for food programs it administers. Canada, the second largest food donor, provides about $1 million a day in surplus farm products.

Canada complains of a lack of clarity in the FAO's budget accounting, a weakness in program review and evaluation, a need for better specification of priorities and planning, inadequate coordination with other UN agencies, and insufficient opportunity for review by member governments of the FAO's Technical Cooperation Program.

Canadian officials also hold that Saouma runs a boss-like operation with something of an anti-Western slant.

Such charges are denied by FAO spokesman Richard Lydiker. He points out that government representatives on both a program committee and a financial committee review the activities of the FAO for two weeks each year. He also noted that priorities are set by the governments and usually without dissent, and that the last budget was approved unanimously.

Delegates of the 158 nations belonging to the FAO will vote on the leadership issue by secret ballot at a November conference. A Canadian head count of FAO members has the race literally tied at the moment between Saouma and Mensah.

Mr. Mensah, according to a Canadian External Affairs official, has the support of countries providing some 90 percent of the FAO's budget and food aid, including the United States, Japan, and the Britain. France supports the incumbent.

Mensah, who is assistant president of the International Fund for Agricultural Development, gets his most solid support from the 42-nation African bloc at the UN.

The efforts to replace Saouma are part of a broader drive by Canada and a few other mostly Western nations to ``reform'' the UN agencies, some of which which they consider inefficient and at times too political.

The US and Britain withdrew from the UN Educational, Scientific, and Cultural Organization in 1984 and 1985, respectively, for these reasons. Canada remained in UNESCO to help reform it from within.

This spring in Geneva, Canada informally submitted a 150-page document outlining possible financial and budgetary reforms for UN agencies to the annual meeting of high-level officials who represent Western nations in UN organizations. One proposed reform would permit director-generals of the agencies to serve only two terms.

During a recent interview here, Mensah talked about his goals: making better use of the FAO's financial and other resources, involving member nations more in deciding priorities, making the budget more understandable to national delegates, and using nongovernmental organizations to a greater extent in executing food programs.

Such reforms, Mensah argues, could encourage more generous giving to the FAO from donor nations.